This invention relates to a lens-applied optical fiber connector, and particularly to an optical fiber connector which is suitable for single mode fibers.
Various kinds of optical connectors for connecting optical fibers have been developed and these can be roughly classified into the direct connecting type in which the optical fibers are arranged directly face to face and the lens-applied type using optical lenses.
The direct connecting type is structured so that the ends of optical fibers are inserted into a sleeve and this sleeve is further inserted into an adapter, so that the optical fibers are arranged face to face. In such a structure, it is essential to make the deflection of the optical axis of the face to face optical fibers small in order to make the connecting losses small. For this purpose, the clearance between the sleeve and adapter should be small, the outer diameter of the sleeve should be formed with high accuracy, an optical fiber insertion hole to be provided in the sleeve should be formed on the center axis of the sleeve and the clearance between the hole and the optical fiber should be small.
In particular, for the single mode fiber, the tolerance of the external diameter of the sleeve should be within 0.5 .mu.m, displacement of the hole should be within 0.9 .mu.m and the play between the hole and the fiber should be within 0.5 .mu.m, but in practice the deflection of the optical axis of the optical fiber must be within 1 .mu.m when all of these factors are taken into consideration. Thus, experiments are conducted for this purpose.
For example, three cylinders of different diameter having a hole at the deflected position are combined in order to set the optical fiber at the center of the sleeve, and these cylinders are rotated independently. Then, a ball bearing is provided at the adapter in order to make the clearance between the sleeve and adapter small. In this manner the sleeve is inserted tightly.
In another example, a split sleeve is provided within the adapter and the sleeve is inserted tightly.
In these structures, the sleeve is inserted tightly with a large inserting or removing force in order to make the clearance between the adapter and sleeve small.
Moreover, such a press-in apparatus results in the wear of the sleeve and adapter and since the clearance between fibers is as small as 1 to 2 .mu.m, friction powder adheres to the end surface of the optical fibers, resulting in a connecting loss.
In addition, the structure for positioning an optical fiber at the sleeve center is complicated and requires a long period of time for adjustment. With such a background, a lens-applied optical fiber connector utilizing a couple of lenses between the optical fibers has been proposed.
In the lens-applied connector, the lenses and optical fibers are arranged in such a way that the distance between the lens and optical fiber surface becomes equal to the focal distance of the lens. As a result, the light beam emitted from one optical fiber is diverged in parallel by the lens and then converged again by the other lens and focused at the end surface of the other optical fiber. In such a lens-applied connector, the required accurancy for deflection due to divergence of light is alleviated to some extent.
Therefore, the press-in of the sleeve into the adapter which is done in the direct connecting type of connector is avoided but a disadvantage is that the loss becomes large compared to the direct connecting type when an angular deflection of the optical fiber is generated, namely when the optical axes of two optical fibers to be connected are inclined with respect to each other.